Orthogonal frequency-division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) wireless communication systems under IEEE 802.16 use a network of base stations to communicate with wireless devices (i.e., mobile stations) registered for services in the systems based on the orthogonality of frequencies of multiple subcarriers and can be implemented to achieve a number of technical advantages for wideband wireless communications, such as resistance to multipath fading and interference. Each base station (BS) emits and receives radio frequency (RF) signals that convey data to and from the mobile stations. For various reasons, such as a mobile station (MS) moving away from the area covered by one base station and entering the area covered by another, a handover (also known as a handoff) may be performed to transfer communication services (e.g., an ongoing call or data session) from one base station to another.
Three handover methods are supported in the mobile WiMAX specification; one is mandatory and other two are optional. The mandatory handover method is called the hard handover (HHO) and is the only type required to be implemented by mobile WiMAX initially. HHO implies an abrupt transfer of connection from one BS to another. The handover decisions may be made by the MS or the BS based on measurement results reported by the MS. The MS may periodically conduct an RF scan and measure the signal quality of neighboring base stations. The handover decision may arise, for example, from the signal strength from one cell exceeding the current cell, the MS changing location leading to signal fading or interference, or the MS requiring a higher Quality of Service (QoS). Scanning is performed during scanning intervals allocated by the BS. During these intervals, the MS is also allowed to optionally perform initial ranging and to associate with one or more neighboring base stations. Once a handover decision is made, the MS may begin synchronization with the downlink transmission of the target BS, may perform ranging if it was not done while scanning, and may then terminate the connection with the previous BS. Any undelivered Protocol Data Units (PDUs) at the BS may be retained until a timer expires.